U.S. Pat. Nos. 3,394,068, 3,788,959 and 3,844,927 describe a method for electrodialytic, separation of mixtures, regeneration of acids containing metallic ions, and more particularly to a method for regenerating spent pickle liquor and recovering magnetic oxides and hydroxides of iron therefrom. The process as described in U.S. Pat. No. 3,788,959 utilizes an electrodialytic cell having an end anode chamber adapted to contain anolyte an anode having a surface in the anode chamber adapted to be in contact with the anolyte, an end cathode chamber adapted to contain catholyte, a cathode having a surface in the cathode chamber adapted to be in contact with the catholyte, a spent liquor chamber adapted to contain a spent liquor, such as, an acid containing metallic ions, and at least one concentrating chamber adapted to contain an electrolyte and disposed between the spent liquor chamber and the anode chamber, all of said chambers being separated from adjacent chambers by ion permeable membranes. The chambers are incorporated within a suitable frame and clamping means and means for imposing a difference in potential across the anode and cathode are provided as well as means for passing electrolytes, liquors and the like into, through and out of the respective chambers. When the chambers of the described apparatus contain the respective electrolytes, liquors or solutions, and a potential is provided across the electrodes, the process of the invention occurs. The acid is regenerated, recovered and concentrated in the anode and/or concentrating chambers, and the metallic ions form insoluble product or precipitate in the spent liquor chamber.
The electrodialytic process of said invention for treating spent liquors to separate metal values from acid values in said liquors comprises passing a first feed stream of spent liquor, into, through and out of a spent liquor chamber defined by a first and second ion permeable membrane, the first ion permeable membrane being an anion permselective membrane and the second ion permeable membrane being characterized by its ability to permit the migration of anions. A catholyte solution is circulated in the cathode chamber on the opposite side of the first ion permeable membrane. A second feed stream of an electrolyte solution is passed into, through and out of at least one concentrating chamber provided on the opposite side of the second ion permeable membrane. The concentrating chamber or chambers is defined by the second ion permeable membrane and a third ion permeable membrane. An anolyte solution is circulated in the anode chamber on the side of the third ion permeable membrane opposite the acid electrolyte solution. When a direct current is applied to the apparatus, regenerated and excess acid is recovered and concentrated in the acid concentrating chambers, and in certain embodiments of the invention, in the anode chamber. Metal values precipitate out in the spent liquor chamber and may be recovered externally in a suitable solids removal device. The catholyte electrode reaction generates hydroxyl ions that will neutralize free acid and then form substantially insoluble product when they migrate into the presence of hydrogen and then metallic ions in the spent liquor within the spent liquor chamber.
In accordance with said invention it is believed that anions in the spent liquor migrate into the electrolyte solution in the concentrating chamber and associate therein with cations having high mobility, such as, hydrogen ions, and when the cations having high mobility are hydrogen ions, an acid is formed in said electrolyte thereby enriching it with acid values. Anions from the catholyte migrate through the first ion permeable membrane into the spent liquor and there associate with hydrogen ions in the mixture when the mixture contains excess free acid. When hydrogen ions in the spent liquor associate with hydroxide ions which have migrated from the catholyte, they form water in the mixture. If the spent liquor contains no excess free acid or insufficient free acid to provide sufficient hydrogen ion to associate with the migrating catholyte anions, the migrating catholyte anions, generally OH-ions, form highly separable insoluble products with metal cations in the spent liquor.
A sequestrant is supplied to the catholyte solution to prevent the formation of insoluble compounds within the matrix of the anion permeable membrane which separates the catholyte solution from the spent liquor.
However in practicing this invention, at the onset of iron oxide formation at the initial stages iron oxides, probably magnetite, form in the cathode membrane surface in the pickle liquor chamber side, this finally leads to the formation of metallic iron on this membrane surface at the pickle liquor interface. This phenomenon causes membrane pore blockage and build up of high electrical resistance, resulting into non-operation (or stoppage) of the operation.
It has been further noted that as gross iron oxide and magnetite formation starts during the continued operation of the process the same membrane will be coated with these solids in spite of highest fluidization flow rates of the liquids through the pickle liquor (Henceforth abbreviated as PL) chamber. However, during this phase of the process, which continues until the practical limits of pickle liquor depeletion is achieved, metallic iron is again formed but not on the catholyte PL membrane surface but on the surface of any iron oxide or magnetite sticking to the membrane surface on the anode side. The accumulation of the sticky iron oxide deposit causes increasing electrical resistance in that chamber. Probably the metallic iron formation starts when the resistance becomes high enough to induce reduction of Fe++ to Fe metal on the oxide surface facing the acid chamber. High resistance prevents passage of current through the process chambers causing the operation to decrease markedly.
Accordingly, it is an object of this invention to provide a process which would regenerate spent pickle liquor and recover magnetic oxides and oxides of iron therefrom without plugging or fouling the membrane or the pickle liquor chamber.
It is another object of this invention to provide an apparatus which would permit fluidized flow of the iron oxide formed so that it can be easily removed from the chamber for external separation.
This invention proposes to overcome these prior art problems by forcing the PL stream through the chamber at fluidization turbulent flow velocities by means which imparts vibration to the flowing streams and thence to a vibratable membrane (thin or pliable separating the PL and catholyte chambers). To promote high fluid flow velocities in the turbulent flow range the interior of the pickle liquor frame is specially formed to produce a tortuous path design. It has also been found that it is essential that additives such as certain chelating agents or sequestering compounds be added to the PL chamber rather than to catholyte as is known in the prior art. This addition of chelating agent in the pickle liquor stream eliminates the increase of unusual electrical resistance in the pickle liquor process. A complete understanding of the invention may be obtained from the following description and explanation which refer to the accompanying drawing, illustrating an embodiment for spent pickle liquor regeneration.